Generally, main transmitters and repeaters are set up according to surrounding geographical features/objects and service areas. A repeater is set up in an area where signals from a main transmitter are received weakly to thereby resolve a problem of the area with trouble in signal reception, which is referred to as a weak-signal reception area, and broaden the coverage of the signals transmitted from the main transmitter.
FIG. 1 is a diagram illustrating a service using conventional repeaters in accordance with an embodiment of a prior art, where repeaters repeats signals by using different frequencies.
In the service using conventional repeaters as shown in FIG. 1, first, a signal is transmitted from a main transmitter 101 through a transmission frequency A and repeaters 102 to 105 repeat the signal in frequencies B, C, D and E which are different from the transmission frequency A. However, since the repeaters of FIG. 1 resolve the problem of a weak-signal reception area where the signal from the main transmitter 101 is received weakly and broaden the service coverage by giving different frequencies B, C, D and E to the respective repeaters 102 to 105, the repeaters 102 to 105 use a plurality of frequency bands, and this requires many frequency resources. Thus, it is quite inefficient in the respect of using frequencies.
FIG. 2 is a diagram illustrating a service using conventional repeaters in accordance with another embodiment of a prior art. It presents a conceptual view of a service using on-channel repeaters which repeat signals in the same frequency. In other words, the main transmitter 201 transmits a signal through a transmission frequency A, and the repeaters 202 to 205 repeat the signal through the same frequency as the transmission frequency A. To provide the service, a receiver should be able to discern signals transmitted from the main transmitter 201 and the on-channel repeaters 202 and 205.
Generally, a receiver includes an equalizer for correcting signal distortion in a transmission channel by equalizing the received signal. The equalizer of the receiver can remove a signal that is received after temporally delayed other than a desired signal in the same frequency band.
Thus, the service using the on-channel repeaters requires a precondition that the time delay between signals transmitted from the main transmitter and the on-channel repeaters should be small. That is, the time delay of the on-channel repeaters should be minimized.
Meanwhile, on-channel repeaters generally repeat an inputted radio frequency (RF) signal through an on-channel by demodulating the inputted RF signal into a baseband signal to remove noise and modulating the noise-free signal into the RF signal again.
FIG. 3 is a block diagram describing a modulating apparatus of a conventional on-channel repeater.
As illustrated in FIG. 3, the modulating apparatus of a conventional on-channel repeater includes a baseband signal configuring unit 310 for configuring a baseband signal by combining a field and segment sync signal, a pilot adding unit 320 for adding a pilot signal to the above configured baseband signal, an up-sampling unit 330 for up-sampling the baseband signal with the pilot signal added thereto, a filtering unit 340 for converting the up-sampled baseband signal into an in-phase (I) signal and a quadrature (Q) signal and performing filtration with a Square Root Raised Cosine (SRRC) filter, an intermediate frequency (IF) up-converting unit 350 for up-converting the filtered baseband I and Q signals into I and Q signals of an IF band, an adding unit 360 for adding the up-converted IF band I and Q signals, and a Digital-to-analog converter (DAC) 370 for converting a digital IF signal into an analog IF signal.
Herein, the filtering unit 340 is formed of an I filter [g(n)·cos(2π·f·nT)] 341 and a Q filter [g(n)·sin(2π·f·nT)] 342. Herein, the frequency f is 5.38 MHz and g(n) denotes, a transfer function of the SRRC filter.
Also, the IF up-converting unit 350 is formed of an I signal up-converter 351 to be multiplied by cos(2π·fIF·nT) and a Q signal up-converter 352 to be multiplied by sin(2π·fIF·nT). Herein, the frequency fIF is a frequency that up-converts the frequency of a filtered signal into the IF band frequency.
Meanwhile, a signal generated by a filtering unit 340 of FIG. 3 should satisfy a spectrum standard which is called Spectrum Mask. When an up-sampling rate is 4, the filtering unit 340 using an SRRC filter can satisfy the spectrum standard only when it uses an SRRC filter of more than 500 taps.
Herein, the time delay caused by the filtering unit 340, which is a time delay device, is determined based on the number of used filter taps. For example, when it is assumed that the I filter 341 and the Q filter 342 which form the filtering unit 340 have N taps and M taps, respectively, the I filter 341 and the Q filter 342 generate time delay of N/2 and M/2, respectively.
If the I filter 341 and the Q filter 342 are all SRRC filters, the two filters 341 and 342 have the same tap number and, since the I filter 341 and the Q filter 342 has a parallel structure, the total time delay caused by the filtering unit 340 is as much as N/2 (or M/2). After all, a filtering unit 340 using an SRRC filter having more than 500 taps to satisfy the spectrum standard generates time delay of 250 (=500/2). Since the time delay goes out of the time delay removal capability of the equalizer in the receiver, the receiver cannot discern the output signal of the main transmitter from the output signals of the on-channel repeaters.
Therefore, it is desperately needed to develop a modulating apparatus having a short time delay between the output signal of the on-channel repeaters and the output signals of the main transmitter, that is, a modulating apparatus that can reduce the time delay of the on-channel repeaters.